1. Field of the Invention
This invention relates to a process of preparing cyclodextrins or other compounds capable of producing clathrates with a gas.
2. Description of Prior Art
Cyclodextrins are cyclic oligosaccharides obtained by enzymatic conversion of starch. They correspond to the general structure (C.sub.6 H.sub.10 O.sub.5 /n). The three most current cyclodextrins designated .alpha., .beta. and .gamma. respectively comprise 6, 7 and 8 glucose units.
The cyclodextrins have the remarkable property of being able to form inclusion complexes or clathrates with a wide variety of chemical compounds (see Cyclodextrins and their inclusion complexes by J. SZEJTLI, AKADEMIAI KIADO, Budapest 1982).
The association between the compound and the host molecule is based, according to certain authors (see S. M. Hagan, Clathrate Inclusion Compounds, Reinhold Publishing Corporation 1962, p. 23), on the multiple Van der Waals bonds which are formed between the two compounds.
Inclusion may take place in solution or in solid state. However, the inclusion of a molecule in cyclodextrins is generally carried out starting from an aqueous solution or suspension.
The inclusion complex is generally less soluble than cyclodextrin. The inclusion may therefore lead to the precipitation of crystals of complexes also called clathrates.
By placing the clathrate back into solution or by simple heating, the inserted substance may be released.
Calthrates of gases have been known since 1957 (F. CRAMER and F. M. HENGLEIN, Chem. Ber. (1957), 90, 2561-2571, and 2572-2575).
Most gases lead to the formation of inclusion complexes when they are in contact with a solution of cyclodextrin. However, the production of crystals is not always observed. This is the case, for example, for oxygen, nitrogen and carbon monoxide. On the other hand, chlorine, carbon dioxide, nitrous oxide (N.sub.2 O), xenon, krypton as well as all the light hydrocarbons (CH.sub.4, C.sub.2 H.sub.2, C.sub.2 H.sub.4, C.sub.2 H.sub.6, etc . . . ) give clathrates ( see: J. SZEJTLI, Cyclodextrin Technology, Kluwer Academic, Publishers, 1988, p. 165-166).
The cyclodextrins and their acylated, alkylated and glucosilated derivatives, are used for the inclusion of active principles or aromas and perfumes in the pharmaceutical and cosmetic fields, respectively.
A Japanese patent publication [JP62039602 (1987)] describes the production of beta-cyclodextrins complexes and derivatives thereof with carbon dioxide, and their exemplary utilization in the cosmetic field.
Clathrates of the above described gases may be prepared from solutions of cyclodextrins which themselves have been obtained by enzymatic conversion of starch.
The isolation of cyclodextrins as well as their purification was the object of a plurality of works and publications (Dominique DUCHENE, Cyclodextrins and their Industrial Uses, Editions Sante, 1987).
The direct crystallization of cyclodextrins issued from the medium used for the enzymatic conversion of starch gives bad yields. Consequently, many complicated and costly techniques of purification have been developed, such as, for example, chromatography (adsorption on resins or gel).
An appropriate solvent may also be used. In this case, cyclodextrin is precipitated in the medium in the form of cyclodextrin/solvent complex, and it is possible to separate the cyclodextrin from the medium by a known technique, for example, by filtration and centrifugation.
These chromatographic or precipitation techniques mentioned above have substantial disadvantages mainly connected with the use of the solvent. These processes require specific and costly apparatuses for handling the solvent because of its toxicity and flammability, as well as its recycling.
Moreover, the presence of solvent residue in the cyclodextrins is harmful for applications of cyclodextrins, for example, in the pharmaceutical field.